In Order for Mutually Beneficial Trade to Occur Between Two

In order for mutually beneficial trade to occur between two otherwise isolated nations,

A) each nation must be able to produce at least one good absolutely cheaper than the other.
B) each nation must be able to produce at least one good relatively cheaper than the other.
C) each nation must face constant costs in the production of the good it exports.
D) one nation's production must be labor-intensive, while the other nation's production is capital-intensive.